The present invention relates to static converters and more particularly to a trigger circuit system and method for controlling the firing angles for controlled rectifiers for converting between multiphase a-c power and d-c power and for providing a regulated output therefrom.
Many industrial systems utilize voltage and/or current converted from a-c or d-c energy. In the description which follows a conversion system from a-c to d-c is described; it should be understood that the principles are equally applicable to the reverse i.e. d-c to a-c.
Thus the d-c energy in many cases is generated from a three phase a-c input by static converters utilizing controlled rectifiers. In order to maintain a constant d-c output (voltage and/or current) and to compensate for load fluctuations, variations in a-c input voltage, etc., it is necessary to vary the phase angle at which the controlled rectifiers conduct. This is done by varying the time at which the trigger pulses, which fire the controlled rectifiers, are generated. At the same time since a three phase a-c input is used, the trigger pulses must occur in proper sequence and time for each half cycle. Thus the trigger pulses will be occurring generally sixty (60.degree.) electrical degrees apart and six trigger pulses are required for each complete three phase cycle. Unless the timing of the trigger pulses is closely controlled d-c voltage and/or current control problems can occur. Some of the problems and proposed solutions therefor have been presented in a paper by J. D. Ainsworth, Proceeding, IEE, Vol. 114, No. 7, July 1967; another solution is presented in U.S. Pat. No. 3,648,078 to James H. Galloway, issued on Mar. 7, 1972.
In the present invention a circuit is operative with a microcomputer to control an internal (computer) counter which operates from a system clock. The internal counter is set to a number (trigger sequence number) which in a timed system is representative of the number of electrical degrees between successive pulses which will provide the desired phase angle at which firing of each controlled rectifier for the related one of the three phase inputs will occur and, with associated circuitry, is effective to provide a trigger pulse each time the sequence number is reached. This number is periodically updated and hence varied in magnitude to accommodate variations in load, etc., whereby the proper phase relationship of trigger pulses and associated line phase is provided to respond to load variations, line fluctuations, etc., in order to maintain a constant d-c (voltage and/or current) output. At the same time the sequence of occurrence of the trigger pulses is controlled to provide firing in accordance with the rotation of the phases of the three phase input.
Therefore, it is an object to provide a new and improved trigger circuit and system for controlling the firing angles for controlled rectifiers in a multi-phase static converter.
It is another object to provide such a circuit and system utilizing a circuit in which the time for occurrence of trigger pulses is determined by a predeterminable digital number and furthr in which the magnitude of the predeterminable digital number is varied to maintain the proper and desired phase relationship between trigger pulses and the associated phases of the three phase input whereby the d-c output is regulated.